Shape memory polymer actuators

ABSTRACT

Resistive heating elements are embedded in a shape memory polymer actuator. Sensing elements are associated with the resistive heating elements. The sensing elements sense changes in the resistive heating elements and correlate the changes with deformation of the shape memory polymer actuator.

FIELD OF THE INVENTION

The present invention generally relates to shape memory polymeractuators, such as for space applications and their durability in thelow earth orbit space environment.

BACKGROUND OF THE INVENTION

Shape memory polymers (SMPs) are smart materials with an ability torecover their original (permanent) shape from a deformed (temporary)shape, by applying an external stimulus, such as elevated temperature.In space applications, due to their high strength-to-weight ratio andlarge deformability, SMPs can be used as deployable devices and replacetraditional heavy metal-based mechanisms. The low Earth orbit (LEO)space environment includes hazards such as atomic oxygen (AO), UVradiation, ultrahigh vacuum (UHV), severe temperature cycles, andorbital debris. Exposure of SMPs to LEO environment might result indetrimental effects such as erosion, discoloration, and outgassing. Thedamage may be enhanced by the synergetic effect of space environmentcomponents; hence, materials such as SMPs must be protected andqualified for the space environment through ground-based simulation.

Most satellites are being launched into low Earth orbit (LEO) altitudes,from 200 to 800 km [1, 2]. The LEO environment involves severeconditions, such as atomic oxygen (AO), ultrahigh vacuum (UHV),micro-meteoroid and space debris impacts, ionizing ultraviolet (UV) andvacuum UV (VUV) radiation, electrostatic discharge, and thermal cycling±100° C. In order to ensure satellites survival in LEO environment, itis necessary to understand the environmental effects on the satellitematerials. The most destructive constituents for materials in LEO spaceapplications are AO and UV radiation [1, 3, 4]. Under the LEO severeconditions, the use of advanced, durable, and lightweight materials isneeded. Shape memory polymers (SMPs) are good candidates for spaceapplications, mainly due to their high strength-to-weight ratio andtheir ability to replace heavy metal-based mechanisms [5]. SMPs arestimuli-responsive materials that, after being deformed, have theability to return to their pre-deformed shape by application of anexternal stimulus, such as light, heat, electric or magnetic fields, pHlevel, or ionic strength. The shape memory material “remembers” itsprevious shape [6-8]. The shape memory effect (SME) in SMPs results froma combination of the polymer structure and morphology together with theapplied processing and programming technology [9]. SMPs are elasticpolymer networks that underlie active movement. The polymer networkconsists of the molecular switching segment and net-point hard segments.The net-points determine the permanent shape of the polymer network andcan be of either chemical (covalent bonds) or physical (intermolecularinteractions) nature. The molecular switches are able to reduce theirstiffness with a particular stimulus, allowing the polymer to beprogrammed into its temporary shape. Upon exposure to a specificstimulus, the molecular switches are triggered, and strain energy storedin the temporary shape is released, which consequently results in shaperecovery [10, 11]. In the past, research has mainly focused onthermoplastic SMPs. Unfortunately, the structures made of thermoplasticSMPs lose their SME after several cycles. Therefore, thermoset SMPs withhigh material stiffness, high transition temperature (>˜100° C.), andgood environmental durability are becoming the potential selection forthe production of space structures [12, 13]. Thermoelectric-triggeredSMPs are promising candidate materials for space applications as othertriggering mechanisms such as pH, humidity, etc., are not relevant inthe LEO environment. One of the most attractive thermoelectric-triggeredSMPs is epoxy adhesive [12]. Epoxy-based SMPs are favorable for spaceapplications due to their low outgassing properties, high triggeringtemperature, as well as their high strength-to-weight ratio [13, 14].Epoxy SMPs exhibit, for example, shape recovery ratio of 98-100%, and anelastic modulus of 2-4.5 GPa. In addition, they perform well whenexposed to space radiation. Epoxy can be used as a matrix reinforced bycarbon fibers for composite applications such as hinges, solar arrays,deployable panels, booms, and reflector-antennas [10]. Epoxy also hashigh resistance to wear; its surface is relatively hard due to itsaromatic segments. In addition, it has high adhesion to metals due toits polarity. Reinforcement with graphite or carbon particles or fiberscan improve its strength and stiffness [7, 8].

However, the effect of the LEO environment on the deployment kineticsand control of epoxy-based SMPs reinforced with carbon is not fullyunderstood, nor is their durability to the various constituents of theLEO environment, such as AO, or the effect of vacuum conditions on itsdeployment.

During the service of a spacecraft, AO might interact with the SMP andaffect its properties [15]. Polymers containing silicones are moreresistant to AO due to formation of a SiO₂ passivation layer thatprotects the underlying polymer [9]. In previous works, a nanocompositecomposed of polyhedral oligomeric silsesquioxane (POSS) additivescopolymerized or blended with polyimide (PI) demonstrated the formationof a SiO₂ passivation layer as a result of interaction with AO that ledto two orders of magnitude reduction in its erosion yield [16-18].

SUMMARY OF THE INVENTION

One non-limiting embodiment of the present invention includes resistiveheating elements embedded in a shape memory polymer actuator. Sensingelements are associated with the resistive heating elements. The sensingelements sense changes in the resistive heating elements and correlatethe changes with deformation of the shape memory polymer actuator.

During deformation of the SMPA the number of the inter-resistive heatingelement contact-points within the bundle, as well as the density of theπ electrons in the case of carbon fiber-based resistive heater elements,changes. As a result, the electrical resistance changes accordingly. Bymeasuring the electrical resistance of the resistive heating elements,the amount of deformation the SMPA is experiencing can be monitored.Furthermore, the power supply system of the SMPA can use the SMPA'selectrical resistance output as a feedback to control the degree ofdeformation of the SMPA.

One non-limiting embodiment of the present invention also involves thecoating of the SMPA with a thin metallic layer. Thermally activated SMPAloses heat through convection and radiation to the surroundingenvironment. However, by coating the SMPA with a thin metallic layer, itpreserves its temperature by internal radiative heating process whichsignificantly decreases the heat loss. Hence, shorter time and lowerpower are needed to heat the SMPA. In this process, the emitted photonsare reflected back to the bulk polymer by the solar reflective thinmetallic layer. In this manner, the metallic layer preserves thetemperature as a solar-reflective element and reduced the SMPA'selectric power consumption.

The present invention seeks to provide SMPAs that are based on an epoxymatrix and embedded carbon resistive heating wires, as well asPOSS-epoxy nanocomposite SMPs. The deployment kinetics of the SMPAs wasmeasured by the inventors under various environmental conditions; in onenon-limiting embodiment, the invention provides a novel method for theimprovement of the energetic efficiency for SMPAs' deployment using analuminum coating. Means to control the SMPAs' recovery angle throughelectrical resistance adjustment is demonstrated, and a phenomenologicalmodel, which explains the findings, is suggested, although the inventionis not limited to this model or any other explanation. The durability ofthe SMPAs in ground-based simulated AO environment was also studied.Methods to improve their AO durability through incorporation of POSSmolecules were demonstrated, creating a novel SMPA self-passivatingnanocomposite with enhanced durability to the LEO environment.

SMP actuators (SMPAs) that are based on epoxy matrix and carbonresistive heating wires were developed. Their thermal, mechanical, andelectrical properties, as well as their deployment kinetics, werestudied. A novel method for the improvement of the SMPAs' deploymentenergetic efficiency, based on an aluminum coating for internalradiative heating, was introduced. Durability improvement of the SMPAsto AO attack was achieved by copolymerization with Polyhedral OligomericSilsesquioxane (POSS) additive, forming an AO self-passivating novel SMPnanocomposite. Finally, a method to control the SMPA deployment via insitu electrical resistance measurements was demonstrated.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIGS. 1A-1D are prior art illustrations of molecular structure of:

FIG. 1A: EPON 826 resin; FIG. 1B: JEFFAMINE D230 Polyetheraminecrosslinker; FIG. 1C: N-Phenylaminopropyl AM0281 POSS cage; and FIG. 1D:schematics of a carbon fiber structure [19].

FIGS. 2A-2C are illustrations of (FIG. 2A) epoxy-carbon SMPA (includinga resistive heater with a w-like architecture), (FIG. 2B)aluminum-coated epoxy-carbon SMPA, and (FIG. 2C) epoxy-reference andPOSS-epoxy samples.

FIGS. 3A-3B are illustrations of (FIG. 3A) bending by INSTRON machine at130° C., and (FIG. 3B) recovery parameters superimposed on an SMPAinstalled in a vacuum chamber.

FIGS. 4A-4D are illustrations of macroscopic SME by resistive heating atambient pressure conditions: FIG. 4A: Beginning of test, SMPA in itstemporary shape; FIGS. 4B and 4C: After 3 and 6 min, respectively, fromthe beginning of the resistive heating, and FIG. 4D: Full deployment topermanent shape.

FIGS. 5A-5D are illustrations of: FIG. 5A: 1^(st) stage results,resistance versus temperature during oven heating of the SMPA; FIG. 5B:2^(nd) stage results, resistance versus deflection during bending of thepreheated SMPA at 130° C.; FIG. 5C: 3^(rd)-stage results, resistanceversus temperature during cooling of the SMPA; FIG. 5D: 4^(th) stageresults, resistance versus deflection during recovery of the SMPA byresistive heating.

FIGS. 6A-6E are illustrations of: FIG. 6A: SMPA after bending andcooling; FIG. 6B: Buckling of the carbon fibers due to compressionstress; FIG. 6C: The SMPA after recovery; FIGS. 6D-6E: Micro- andnano-meter scale HRSEM images of carbon fibers, respectively.

FIG. 7 is an illustration of a phenomenological model, whichdemonstrates the major parameters that influence the SMPA's electricalresistance during bending, on the micro- and macro-structure levels.

FIGS. 8A-8D are illustrations of recovery of the SMPAs at ambient andvacuum conditions, with and without aluminum (Al) coating, wherein FIG.8A shows recovery angle change (Δθ) versus deployment time, FIG. 8Bshows Δθ versus deployment power, FIG. 8C shows angular velocity versusdeployment time, and FIG. 8D shows resistance versus Δθ.

FIGS. 9A-9D are illustrations of SME of 15 wt. % POSS-epoxy SMP samplein its (FIG. 9A) initial permanent shape, before SME; (FIG. 9B)temporary shape; and (FIG. 9C) final permanent shape, after SME; andFIG. 9D shows samples' mass loss versus LEO equivalent AO fluence.

FIGS. 10A-10E are illustrations of: SME of 15 wt. % POSS-epoxy SMPsample in its (FIG. 10A) initial permanent shape, before SME; (FIG. 10B)temporary shape; and (FIG. 10C) final permanent shape, after SME; andFIG. 10D shows samples' mass loss versus LEO equivalent AO fluence HRSEMimages of (a, c) epoxy reference and POSS-epoxy SMPs, respectively,before exposure to AO, and (b, d) epoxy reference and POSS-epoxy SMPs,respectively, after exposure to LEO equivalent AO-fluence of 1×10²⁰O-atoms/cm²; and FIG. 10E shows a high-resolution image of thePOSS-epoxy SMP after exposure to LEO equivalent AO.

DETAILED DESCRIPTION OF EMBODIMENTS

Adhesive for the SMPA was prepared from EPON 826 DGEBA resin (Momentive,Inc.) and JEFFAMINE D230 Poly(propylene Glycol)bis(2-Aminopropyl) Ethercrosslinker agent (Huntsman Chemicals, Inc.). AM0281 N-PhenylaminopropylPOSS cage mixture additive (Hybrid Plastics, Inc.) was mixed with theamino-based crosslinker to create 15 wt. % POSS-epoxy actuator. Thechemical structures of these materials are shown in FIGS. 1A-1D. Inaddition, 3k-70-p, type 2, grade 3, class 1 polyacrylonitrile(PAN)-based carbon fiber yarn (Hexcel Inc.) was used.

Four types of epoxy-based SMPAs were prepared using an aluminum mold, inwhich carbon fibers were immersed into the cavity, and epoxy adhesivewas poured above. The materials used for the various SMPs and SMPAsprepared in this work are summarized in Table 1.

Epoxy resin was mixed with the crosslinker agent in a volume ratio of2.52:1, respectively [in accordance with ref. 20]. Both materials werepreheated to 50° C. and added to a vial. The vial was first shakenvigorously by hand, and then by Vortex shaker for 1 minute at 30 rpm.Next, the vial was placed in a vacuum oven, which was preheated to 50°C., for degassing at a pressure of less than 10 mmHg. After 13 minutes,the vial was taken out, and the adhesive was ready to be poured into themold.

TABLE 1 Composition of the epoxy based SMPs and SMPAs. EPON AM0281 826JEFFAMINE POSS Carbon Aluminum SMP/SMPA (wt. %) D230 (wt. %) (wt. %)fiber coating Epoxy- 75.5 24.5 — — — reference POSS-Epoxy 67.6 17.4 15.0— — Epoxy-carbon 75.5 24.5 — ✓ — Aluminum- 75.5 24.55 — ✓ ✓ coatedEpoxy- carbon

Before pouring the adhesive, the molds, having inner dimensions of70×10×1 mm³, were coated with a WATERSHIELD release agent (Zyvax, Inc.).In the next stage, the carbon fibers were connected to electrical wiresand were placed in the mold. Next, 0.8 mL of the adhesive was pouredinto each mold. The epoxy adhesive was thermally cured at 100° C. for1.5 h, and post-cured at 130° C. for another 1 h [in accordance withref. 20]. Upon completion of curing, the mold was cooled to roomtemperature (RT), and the epoxy-based SMPAs were demolded. FIG. 2adepicts the SMPA scheme, including the resistive heaters' w-likearchitecture. Some of the samples were coated with 100 nm thick aluminumcoating, using electron beam physical vapor deposition (EBPVD) at 1 Å/srate, 2×10⁻⁶ Torr, and RT conditions, as schematically illustrated inFIG. 2b . Additional samples of pristine epoxy and POSS-epoxy wereprepared in the same manner, without carbon fibers and without aluminumcoating, see FIG. 2c . The POSS additive was pre-mixed with thecrosslinker at 50° C. A molar ratio of 1:1 was kept between the primaryamine groups of the crosslinker and the amine groups from the POSS thatreplaced them, while the overall POSS content was set to 15 wt. %.

The SMP samples and SMPAs were deformed to a u-like shape using 3-pointbending grips, mounted on an INSTRON 3365 universal machine, equippedwith an environmental chamber and a 100 N load cell. The bending wasperformed by using a 22.8 mm radius “upper nose”, see FIG. 3a . Thissetup was used for fixing the SMPAs in their temporary shape, at atemperature of 130° C., using the environmental chamber, and thencooling to RT while keeping the SMPAs under load. The SMPAs weredeflected to a maximum strain of 2.4% at 0.4 mm/min crosshead speed and42 mm support span. 2.4% strain is equivalent to 9.5 mm deflection,according to Equation 1 [in accordance with ref. 21].

$\begin{matrix}{ɛ = {\frac{6{Dd}}{L^{2}} \times 100}} & (1)\end{matrix}$

where ε (%) is strain, D (mm) is the deflection, d (mm) is the thicknessof the sample, and L (mm) is the support span.

The SME of the SMPAs was measured either at ambient pressure or invacuum (5.5×10⁻⁴ Torr) using resistive heating. The recovery effect wasrecorded by a video camera, and was quantified by the following values:recovery angle (Δθ) and deflection (D), see FIG. 3b , where at t=0,Δθ=0° and D=D_(max).

The surfaces of the SMP samples were characterized by a high-resolutionscanning electron microscope (HRSEM) equipped with a secondary electronsdetector (model Sigma 300 VP from Zeiss). Images of the epoxy referenceand POSS-epoxy samples were collected in a variable pressure mode. Thisenabled measurements of the insulating samples without the applicationof a conductive coating [compare ref. 22].

The durability of the reference and POSS-containing epoxy SMP samples toAO attack was measured by a ground-based AO simulation facility that isbased on a radio frequency (RF) plasma source (Litmas RPS). The sourceoperates at a maximum power of 3 kW, a frequency of 1.7-3.0 MHz, and anO₂ feed. During the experiment, the samples were held in the RF plasmasource vacuum chamber for a total exposure time of 260 h; they wereremoved periodically from the vacuum chamber for measurement of theirmass loss. The mass loss is used to calculate the material's LEOequivalent AO-fluence. The experiment parameters were: pressure of6×10⁻² Torr, power of 810 W, and O₂ flow rate of 12 sccm. Under theseconditions, a current of 20 μA was measured between the sample holderand the ground, using a picoammeter (model 485 from Keithley). Thiscurrent was used to monitor the RF plasma performance.

The LEO equivalent AO-fluence was calculated by measuring the mass lossof a Kapton sample, which was simultaneously exposed to the AO beam,assuming an erosion yield of 3×10⁻²⁴ cm³/O-atoms [23], see Equation 2[in accordance with ref. 17].

$\begin{matrix}{F = \frac{\Delta\; m}{\rho\;{AE}}} & (2)\end{matrix}$

where Δm is the mass loss (g), A is the material's exposed area (cm²), ρis the material's density (g/cm³), F is the equivalent AO fluence(O-atoms/cm²), and E is the erosion yield (cm³/O-atom).

FIG. 4 depicts a visual description of the shape recovery of the SMPA.In this experiment, the SME was obtained by resistive heating underambient pressure conditions. The current was increased gradually at aconstant rate of 0.03 A/min to avoid uneven heat distribution and, as aresult, uneven strains that can lead to local failures. Full recoverywas accomplished after 7 min, using resistive heating current of 0.21 A,see FIG. 4d . The recovery effect started just after the actuatorreached a temperature equal to the glass transition temperature of theepoxy.

SMPA Recovery Control Through Electrical Resistance Measurements

During bending and shape memory recovery cycles of the SMPA, changesbetween the electrical resistance at the permanent and the temporaryshapes, as well as at different temperatures, were observed. Hence, itwas decided to study the influence of bending and temperature on theelectrical resistance of the actuators while separating these variables.This methodology was used to control the recovery angle, and may be usedin future space deployable mechanisms to control a deployment process.In order to investigate the effect of temperature and bending on theactuators' electrical resistance, an experiment was performed in fourstages: 1) heating, 2) bending, 3) cooling, and 4) recovery by resistiveheating. The resistance was calculated using Ohm's and Pouillet's laws,see Equation 3 [in accordance with ref. 24, 25].

$\begin{matrix}{R = {\frac{V}{I} = \frac{\rho\; L}{A}}} & (3)\end{matrix}$

where R (Ω) is the resistance, V (V) is the voltage, I (A) is thecurrent, ρ (Ω·m) is the resistivity, L (m) is the length, and A (m²) isthe cross-sectional area.

FIG. 5a shows the electrical resistance as a function of temperature atthe 1^(st) stage during oven heating of the SMPA in its permanent shape.During heating to a temperature of 130° C., its resistance increased by77%, from 66 Ω to 117 Ω. This increase is composed of two phases. Thefirst shows a relatively small increase in resistance, from 66 Ω to 72 Ωas the SMPA is heated from RT to 110° C. Above 110° C., an abruptincrease in resistance is observed, reaching 117 Ω at a temperature of128° C. This phenomenon has been previously observed for epoxy resinsfilled with carbon fibers at various aspect ratios [refs. 26, 27]. Whilethe first phase depicts the resistance change due to the change in thegraphite fiber metallic conductivity, the second abrupt increase inresistance is associated with mechanical changes in the epoxy resinitself. The actuator's semicrystalline carbon fibers contain unorderedplains of hexagonally arrayed sp² carbon atoms sheets. Electricalcurrent passes through the carbon fiber by π electrons, below and abovethese sheets (parallel to the basal xy plane). The mobility of the πelectrons is similar to the behavior of electron gas in metals [ref.28]. During the first phase, while heating the SMPA in the environmentalchamber, electron scattering increases due to increased thermalvibrations of the atoms, i.e., collisions between the electrons and thephonons, and lattice irregularities, e.g., vacancies [refs. 24, 29],resulting in smaller electron mean free path (MFP) [ref. 28]. The abruptincrease in resistance during the second phase of the heating process isassociated with mechanical changes in the epoxy resin itself. The reasonfor this positive temperature coefficient (PTC) effect is thebreaking-off of inter-fiber contacts. The carbon fibers embedded in theepoxy are composed of micron-sized filaments, with a very small distancebetween them. When the polymer matrix shrinks during the curing andfinal cooling processes, residual compressive strain is stored in thepolymer gap between the fibers, as the carbon fibers were pushedtogether. During heating, as the material becomes rubbery, the thermalexpansion of the epoxy will compensate for the residual strain, and at acritical temperature will lead to the separation of the fibers, thuslowering the number of inter-fiber contacts. Consequently, thetemperature at which the PCT effect occurs is highly dependent on thecuring temperature. In this case, the epoxy was cured at 100° C. and130° C.; therefore the PCT effect started at an intermediatetemperature, and was observed above 110° C. Thus, during the 1^(st)stage, the dominant mechanism that affects the SMPA's resistivity iselectron scattering and reduction of inter-fiber contacts, whichincreases the resistivity during the heating [ref. 25].

According to Equation 3, the resistance may also be affected bydimensional changes in the carbon fiber's length and cross-section area.In order to evaluate the influence of the temperature on theseparameters, the linear expansion was calculated according to Equation 4[in accordance with ref. 30]:

$\begin{matrix}{{\Delta\; L} = {L_{0}{\alpha\Delta}\; T}} & (4)\end{matrix}$

where ΔL is the dimensional change, L₀ is the initial dimension, α isthe coefficient of thermal expansion, and ΔT is the temperature change.

The carbon fiber's coefficient of thermal expansion in its z directionis extremely low and negative, −4.1×10⁻⁷° C.⁻¹ [31]; it is about 2×10⁻⁵°C.⁻¹ in its x or y directions [in accordance with ref. 28]. The lengthof the carbon fiber is 26 cm per SMPA, whereas the typicalcross-sectional area of a single fiber filament is 38.5 μm² [inaccordance with ref. 31]. Therefore, the fiber's longitudinal thermaldimensional change is 13.7 μm, merely 5×10⁻³% change. The thermalexpansion of the fiber's cross-sectional area was also very small, only0.2%. Thus, through the 4 stages of the SMPA recovery controlexperiment, L was set constant, and in stage 1 the cross-sectional areawas considered constant too.

FIG. 5b shows the 2nd-stage results of resistance versus deflectionmeasurements during bending of the SMPA to its temporary u-like shape.An increase in the resistance by 9.1%, from 117 Ω to 128 Ω, wasmeasured. This increase may be due to two possible reasons. First, asufficient number of fractured carbon monofilaments, or disconnection ofnearby filaments at certain points, due to the bending, may lead tolower cross-sectional area for electron current, if those filamentsbecame electrically disconnected [in accordance with ref. 32]. Second,the carbon fiber buckling deformation, which occurred during the bendingtest [in accordance with refs. 33, 34], as shown in FIG. 6a and b . Inthe z-direction (normal to the graphitic basal planes), the spacingbetween the graphite sheets presumably becomes larger due to buckling.Hence, the density-of-state of the π electrons decreased, and higherresistance was measured as the electrons were forced to move a largerdistance from one graphite sheet to the other [ref. 29]. FIG. 6dpresents HRSEM images of the carbon fibers, demonstrating theinter-fiber contact points between nearby filaments. FIG. 6e presentsthe HRSEM image of a single filament composed of interconnected fibrilshaving diameters of 20-50 nm.

FIG. 5c depicts the electrical resistance as a function of temperaturein the 3^(rd) stage during cooling of the SMPA to RT in its temporaryshape. During cooling, the resistance of the bended SMPA decreased by32%, from 128 Ω to 87 Ω. During cooling, one parameter may lead to anopposite effect that may increase the actuator resistivity and, hence,its resistance. This parameter concerns the residual stresses that mayincrease between the matrix and the fibers during cooling due to theshrinkage of the thermoset matrix around the fibers [ref. 35], thusincreasing the buckling effect. However, the actual decrease in theresistance during cooling indicates that another, more dominant,parameter exists. It is assumed that this parameter is the reduction inelectron-scattering events. At lower temperatures, fewerelectron-scattering events occur and, hence, the resistivity andresistance decrease [refs. 24, 25, 28].

FIG. 5d depicts 4^(th)-stage results of resistance versus deflectionmeasurements during recovery of the actuator to its permanent shape bygradual resistive heating. The resistive heating itself is a result ofretarding forces and collisions involving charge carriers, usuallyelectrons [ref. 24]. A drastic decrease in the resistance by 23%, from70 Ω to 54 Ω, after deployment of less than 1 mm was observed. Thisdrastic decrease may be attributed to two factors. First, when the SMPAreached the glass-transition temperature, the residual stresses and thebuckling deformation were released immediately as SME started, leadingto closer graphite aligned sheets, higher density of π electrons and,thus, lower resistivity. Second, instant alternative connecting pointsare formed between the carbon fibers during the deployment of the SMPAin the initial deployment stage, which leads to higher cross-sectionalarea for electron current and, hence, the electrical resistivity andresistance decrease. FIG. 6c demonstrates the disappearance of thecarbon fibers' buckling at the end of the SME. The phenomenon of instantdecrease in electrical resistance during the SMPA's deployment may beused for indication and control of the SMPA deployment process while inspace.

FIG. 7 presents a phenomenological model, which demonstrates the majorparameters that influence the SMPA's resistance during bending, on themicro- and macro-structure levels. This model demonstrates thestructure-property dependency, which leads to the resistance decreaseduring deployment, by using three hierarchies: actuator level, carbonfiber macrostructure level, and graphite sheets molecular level. Theactuator level depicts the permanent and the temporary shapes of theactuator, and the associated buckling effect. The carbon fibermacrostructure level shows the fibers in the aligned and buckledposition. The circles around the monofilaments depict themonofilament-to-monofilament electrical contact points, and therectangles show monofilaments fractured due to bending. In the permanentshape, a larger number of contact points exist between themonofilaments. During bending, the monofilaments are far apart, and someof them are fractured. The molecular level depicts the plains ofgraphite sheets in the carbon monofilament and the π electrons below andabove these sheets. The distance between the sheets becomes larger(L_(temporary)>L_(permanent)) during bending, hence the density-of-stateof the π electrons presumably decreased.

Although heating per se increases the carbon fibers' resistivity, duringrecovery by resistive heating, resistivity decreases sharply. Thedominant parameters during recovery, which lead to this decrease in theresistance, are the increase in the number of the inter-fiber contactsand the density of the it electrons.

The Kinetics of SMPAs in Space-Simulated Conditions

The dominant heat transfer mechanisms are different under ambient andunder vacuum conditions. At ambient pressure, the dominant mechanisms ofheat transfer to the surrounding atmosphere are convection andconduction, while in vacuum the main mechanism is radiation [ref. 36].The influence of the vacuum effect on the SMPAs' deployment kinetics wastested in order to simulate the UHV conditions in LEO. In addition, inorder to improve the energetic efficiency of the SMPAs' deployment, anovel approach was developed, which is based on an internal reflectanceheating mechanism. Implementation of this mechanism was done by coatingthe SMPAs with a 100 nm aluminum coating. Aluminum offers highreflectivity in the near-IR and, hence, can be used as solar reflectorof the IR photons, which are emitted during the resistive heatingprocess.

The SMPAs were recovered in both ambient and vacuum conditions, whilealuminum-coated SMPAs were recovered only in vacuum conditions. FIG. 8presents the recovery kinetic parameters of the SMPAs under both ambientand vacuum conditions, with and without aluminum coating. Each curve isthe average of three experiments. FIG. 8a depicts the SMPAs' deploymentkinetics in terms of recovery angle change (Δθ) versus time. The maximumbending angle was defined as Δθ=0°. FIG. 8b shows the SMPAs' deploymentkinetics in terms of Δθ versus the actuator's power consumption. FIG. 8cpresents the angular velocity (dθ/dt) versus deployment time, while FIG.8d presents the SMPAs' electrical resistance versus Δθ. FIG. 8a-c show ageneral trend in all 3 cases of almost instantaneous deployment thatoccurred after a certain dwell time. The highest values of 310 s dwelltime, 97 s recovery duration, and 2.46 W recovery power were observed atambient pressure for the epoxy-carbon SMPA. Performing the epoxy-carbonSMPA deployment experiment under vacuum conditions resulted in moderatevalues of 224 s dwell time, 72 s recovery duration, and 1.53 W recoverypower. The lowest values of 171 s dwell time, 71 s recovery duration,and 1.17 W recovery power were observed under vacuum conditions by thealuminum-coated epoxy-carbon SMPA. These results prove the benefit ofcoating the SMPAs with aluminum, as deployment power consumption (undervacuum conditions) dropped by 25% as a result of this process.

During the recovery stage, electrical current passes through the carbonfibers and produces resistive heating. In this test, the current was setto produce a temperature of around 130° C., above the glass transitiontemperature of the SMPA. According to Wein's law [37], at thistemperature the emitted heat from the actuator has a typical wavelengthof 7.2 μm. At ambient pressure, the SMPA loses heat through convectionto the surrounding air and, hence, longer time and higher power areneeded to heat it. Under vacuum conditions, the aluminum-coated SMPApreserved its temperature by internal radiative heating process. In thisprocess, the emitted photons were reflected back to the bulk polymer bythe solar reflective aluminum coating. In this manner, the aluminumcoating preserved the temperature as a solar-reflective element andreduced the electric power consumption, as shown in FIG. 8 b.

The results presented in FIG. 8c show that at ambient pressure, theepoxy-carbon had the longest dwell time, lowest angular velocity, andwider velocity distribution. In vacuum, the maximum velocity was higher,and the distribution of the angular velocity was symmetric and narrow.These results indicate that the deployment in vacuum occurred at a shortand specific period of time when reaching the glass transitiontemperature. The deployment was even shorter for the aluminum-coatedepoxy-carbon SMPA. The values of the angular velocity were 2.75°/s forthe epoxy-carbon SMPA at ambient pressure, 3.5°/s for the epoxy-carbonSMPA in vacuum, and 6°/s for the aluminum-coated epoxy-carbon SMPA invacuum.

FIG. 8d shows the electrical resistance versus recovery angle change ofthe SMPAs at ambient and vacuum conditions, with or without aluminumcoating. A sharp decrease in the resistance at the beginning of thedeployment, and a lower decrease during the rest of the deployment, areevident. Furthermore, the highest resistance was obtained under vacuumconditions for the aluminum-coated epoxy-carbon SMPA. The lowestresistance was measured under ambient conditions for the epoxy-carbonSMPA. The decrease in the actuator's resistance was due to the releaseof the buckling deformation and the residual stresses, as analyzedpreviously in Section 3.2. The higher the temperature of the SMPA, thehigher its resistance, as shown in FIG. 5a . The aluminum-coatedepoxy-carbon SMPA operated under vacuum conditions reached the highesttemperature due to internal IR reflections, and therefore showed thehighest measured resistance. The epoxy-carbon SMPA operated in ambientconditions reached the lowest temperature, due to heat losses throughconvection and conduction to the surrounding atmosphere, and thereforeshowed the lowest measured resistance.

To briefly summarize this part, the main heat transfer mechanisms forthe SMPA in ambient condition are convection and conduction to thesurrounding air. In vacuum condition, the main heat transfer mechanismis radiation; hence, heat losses are much smaller. As a result, theenergy consumption in vacuum conditions is also much smaller. Coatingthe epoxy-carbon SMPA with aluminum further decreases its energyconsumption by internal radiation, which further decreases heat losses.Hence, internal aluminum coating can save power during the deploymentprocess in space.

POSS-Epoxy SME and Durability to AO

Incorporation of POSS monomers into the epoxy adhesive bycopolymerization may improve its durability to AO attack by formation ofSiO₂ passivation layer. The AO oxidizes the SiO_(1.5) POSS into SiO₂[16]. Hence, reference and POSS-containing epoxy SMP samples wereexposed to oxygen RF-plasma, which simulates AO irradiation. The SMPsamples were removed periodically from the system for a short period oftime in order to measure their mass loss. Prior to the AO exposure, theSME of the POSS-epoxy sample was verified. The POSS-epoxy SMP sample wasexposed to an elevated temperature of 100° C. in a hot bath, bended to au-like temporary shape, fixed in its temporary shape in a cold bath, andexposed again to an elevated temperature until SME occurred and thesample returned to its permanent shape, see FIG. 9a-c . As shown, thePOSS-epoxy sample fully recovered to its original permanent shape,exemplifying the potential of this nano-composite material system as anSMP.

FIG. 9d depicts the mass loss (Δm) of epoxy-reference and POSS-epoxySMPs during ground-based AO-irradiation, as a function of the LEOequivalent AO fluence. The epoxy reference mass loss under a LEOequivalent AO fluence of 2.6×10²⁰ O-atom/cm² was 3.95 mg/cm². Incomparison, the POSS-epoxy SMP mass loss under the same LEO equivalentAO fluence was only 1.17 mg/cm², a reduction of 70% compared to theepoxy-reference SMP. Furthermore, while the mass loss of theepoxy-reference versus the LEO equivalent AO fluence was linear, themass loss of the POSS-epoxy decreased gradually as the LEO equivalent AOfluence increased. This implies a gradual formation of a SiO₂passivation layer during the AO irradiation [3].

FIG. 10 shows HRSEM images of the surface morphology of epoxy referenceand POSS-epoxy SMPs, before and after AO attack. Before AO attack,smooth surfaces are shown (FIG. 10a and c ) for both epoxy andPOSS-epoxy samples. After AO attack, the epoxy-reference surface waseroded to a carpet-like morphology (FIG. 10b ), while the surface ofPOSS-epoxy is characterized by a sponge-like morphology formedpresumably by the SiO₂ self-passivating network (FIG. 10d and e ) [16],which increased its durability to AO attack.

Unique epoxy-carbon SMPAs were designed as building blocks for futurespace applications. Their SME parameters, electrical properties, as wellas their durability in vacuum and AO environments, were evaluated.During resistive heating and deployment, a rapid decrease in the SMPAs'electrical resistance occurs. This effect is caused by temperature andby the SMPAs' deflection state. During deployment, the carbon fibersabruptly unbuckle, compression stresses are immediately released, andnew electrical connecting points are formed between the fibers.Therefore, the carbon fibers serve two purposes—heating elements as wellas deployment control detectors. The SMPAs' resistance measurement canserve as an important tool for controlling the recovery angle duringdeployment in space.

During resistive heating and deployment of the SMPA, electrical currentpasses through the carbon fibers and produces heat. At ambient pressure,the actuator loses heat mainly by convection and conduction to thesurrounding environment; hence, long time and relatively high power areneeded. In vacuum conditions, the deployment of the SMPA is faster andconsumes less power (because less heat is lost), as the main mechanismof heat transfer to the surrounding environment is radiation. When thenovel aluminum-coated SMPA is deployed in vacuum conditions, the coatingpreserves the SMPA's temperature more efficiently by an internalradiative heating process, as photons are reflected back to the bulkpolymer by the reflective aluminum coating. In this manner, the aluminumcoating can save power during the deployment process.

Copolymerization of POSS monomers with epoxy increases their AO erosiondurability and reduces mass loss during irradiation by 70%. The surfaceof the AO post-irradiated POSS-epoxy is characterized by a sponge-likemorphology. These results indicate the formation of a SiO₂ passivationlayer, which increases the POSS-epoxy SMP durability to AO.

It is important to note that the references mentioned above are onlymentioned for ways to measure values or for reference purposes, and inno way anticipate the invention or make the invention obvious.

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What is claimed is:
 1. An assembly comprising: resistive heatingelements embedded in a shape memory polymer actuator, called an SMPA;and sensing elements associated with said resistive heating elements,said sensing elements configured to sense changes in said resistiveheating elements and to correlate said changes with deformation of saidSMPA.
 2. The assembly according to claim 1, wherein said sensingelements comprise electrical resistance sensors configured to sensechanges in electrical resistance of said resistive heating elements. 3.The assembly according to claim 1, wherein upon deformation of saidSMPA, an amount of contact points at which said resistive heatingelements contact each other changes.
 4. The assembly according to claim1, wherein said resistive heating elements comprise carbon fiber-basedresistive heater elements, and upon deformation of said SMPA, a densityof π electrons of said carbon fiber-based resistive heater elementschanges.
 5. The assembly according to claim 1, comprising a power supplycontroller coupled to said SMPA, and wherein electrical resistanceoutput of said SMPA is coupled to said controller as a feedback tocontrol a degree of deformation of said SMPA.
 6. The assembly accordingto claim 1, wherein said SMPA is coated with a metallic layer thatreflects radiative heat energy emitted by said SMPA back to said SMPA.7. The assembly according to claim 1, wherein said SMPA is part of a lowearth orbit space device.